Also skip test_PopGen_DFDist.py if on old Python 3

See earlier commit 9c16c09806ca4af84f714662e54c9bd3057b0a52 for
test_PopGen_FDist.py to skip the test if on an older Python 3
with http://bugs.python.org/issue16903
Fix Bio.ExPASy.ScanProsite.read in Python 3.

ScanProsite.read fails on Python 3, this fix is backwards compatible
with Python 2.
Merge pull request #184 from luizirber/master

Fix for ScanProsite.read, works with Python 2 and 3